Radio Frequency ("RF") power transistors have many uses in industries ranging from communications to consumer electronics, e.g., for use as high frequency amplifiers. Such transistors are commonly designed for attachment to a printed circuit ("pc") board, to be used in conjunction with other circuit components mounted on the same pc board.
Typically, one or more transistor cells are fabricated on a silicon wafer (referred to as a transistor "chip"). The transistor chip is then attached to an insulating layer, normally a ceramic substrate. The ceramic substrate is itself attached to a mounting flange, and a protective cover is placed over the substrate and transistor chip, thereby forming a component transistor "package." Various electrically conductive (e.g., metal) leads are attached to, and extend away from the package (i.e., outside the protective cover) to connect common terminals of the transistor chip to external circuit elements, e.g., located in a pc board along with the transistor package. For example, in a bipolar junction type transistor, respective electrical leads attached to the package are connected to a base, emitter and collector of the transistor chip.
Single layer pc boards--i.e., where there is a single layer of dielectric material between respective top and bottom surfaces of the board,--typically have a metallic bottom surface which is also a reference ground. This bottom surface is connected, usually with screws or solder, to a (metallic) heat sink, so that the bottom of the pc board and the heat sink have the same ground potential with respect to any circuit elements attached to the pc board.
One of two methods is used to connect the transistor package to ground in a single layer pc board. A first method is to provide a "top side" ground, wherein one or more grounding wires connect respective emitter, base or collector pads of the transistor chip to a "ground plane" (also known as a "ground bar"), which is normally attached to the ceramic substrate adjacent the transistor chip. The ground plane is electrically part of a metal lead frame extending from the package. After the package is mounted to the heat sink, the lead frame may be soldered or otherwise connected (e.g., by a wire clip arrangement) to the top of the respective pc board, wherein an electrical path to ground (i.e., the bottom surface of the board) is provided. Although a ground path is achieved by this method, it is not necessarily of uniform length from each of the common emitter (or base) locations.
Another method for grounding RF transistor packages in a single layer pc board is through the use of plated "via holes" located in the ceramic substrate. In particular, the via holes extend from a top surface of the ceramic substrate to a top surface of the mounting flange and are plated with an electrically conductive material, e.g., metal, thereby allowing current to flow through them. Again, one or more grounding wires connect respective emitter, base or collector pads of the transistor chip to a ground plane. The ground plane is provided directly over, and is in electrical contact with the via holes, so that current passes from the ground plane, through the via holes, to the mounting flange. The mounting flange is then connected, usually by screws, to the metal heat sink, which, as noted above, is an equivalent potential as the board's bottom surface. Proper spacing of the via holes is desirable so that each emitter, base or collector pad of the transistor chip is located approximately the same electrical distance to ground potential, and thereby reduce unwanted inductance.
Of the aforedescribed grounding methods for single layer pc boards, the via holes are generally considered superior from a performance perspective, because the electrical path to ground (e.g., the heat sink surface) is more direct and uniform over the entire transistor device. On the other hand, where performance requirements will allow, the top side "lead frame" method is perferred, as it is less costly to manufacture the packages without the via holes.
With the need to reduce component size, amplifier designers are being forced to develop RF power transistor packages for multi-layer pc boards, wherein proper grounding becomes more difficult. In particular, in a multi-layer pc board, there are at least two, and often more layers of dielectric insulating material between the respective bottom and top surfaces of the pc board, with an additional metallic layer located between two of the dielectric layers. This additional metallic layer acts as ground reference for certain components mounted on the top surface of the pc board. Thus, when the bottom of the board is mated with a heat sink, there are two ground planes:--the metal layer between the dielectric layers of the pc board, and the heat sink. While these two ground planes are connected, components mounted to the top surface of the pc board will still "see" the ground plane of the middle metallic layer much more so than the ground plane of the heat sink surface.
When an amplifier designer is required to use an RF power transistor in a multi-layer pc board whose performance requirements would otherwise require the use of plated via holes for grounding, i.e., in order to minimize common lead inductance over the ground path, there is a problem in that the RF power transistor will use as the predominant ground plane the heat sink, to which it is directly attached, whereas the other circuit elements mounted to the board will use the middle metallic layer as the predominant ground plane.
In particular, the RF power transistor operates at very high frequencies, which can result in an undesirable isolation between the two ground planes. This isolation can prevent high frequency components of the transistor current from flowing from the middle metal layer to the heat sink bottom of the pc board. This, in turn, causes the transistor to "see" a different voltage at the middle metal layer than at the heat sink. Having different reference voltages for various electrical components in the same pc board can lead to instabilities, poor production yields and generally lower reliability.
Thus, it is a desirable objective to provide a grounding method for an RF transistor power package designed for use in a multi-layer pc board and, in particular, with minimal common lead inductance between the package and the supporting circuitry.